1. Field of the Invention
The present invention generally relates to structures and assembling methods for an optical passive device including optical components such as a magneto-optical crystal which has a Faraday effect (hereinafter referred to as a "Faraday component") and a polarizer.
2. Background Art
An optical passive device is a type of optical component interposed in an optical network so that an optical signal transmitted therethrough can be distributed and controlled with respect of its transmission direction. Recently, such devices have been widely put into use as a result of the prevalence of optical transmission lines.
An example of such an optical passive device is an optical isolator that is a device for selectively controlling the passage of a light ray according to the transmission direction of light, by utilizing the Faraday effect, i.e., a rotating effect of a plane of polarization by a Faraday rotator. FIGS. 17(a) and 17(b) illustrate conventional basic structures of optical isolators. In the structure shown in FIG. 17(a), a Faraday component 2 is placed in a hollow portion of a hollow cylindrical magnetic member 1, and polarizers 3 are attached at opposite ends of the magnetic member 1. Though either polarizer is usually called an analyzer, a polarizer and an analyzer are basically the same optical elements. Thus, the term "polarizer" is employed throughout this specification for avoiding awkwardness of distinguishing an analyzer from a polarizer. The Faraday component 2 is supported in the magnetic member 1 and rotates the plane of polarization of an incident light ray by forty-five (45) degrees by the effect of the magnetic field of the magnetic member 1. The combination of the magnetic member 1 and the Faraday component 2 constitutes a Faraday rotator.
As for the structure in FIG. 17(b), a circular yoke 4 is disposed around the Faraday component 2. The yoke 4 is integrally assembled with two cylindrical magnetic members 1a, 1b fastening the yoke 4 therebetween. The polarizers 3 are attached on the ends of the cylindrical magnetic members 1a and 1b, respectively.
FIG. 18 illustrates an optical isolator with holders which support the polarizers 3. The optical isolator 6 includes a hollow cylindrical magnetic member 1, a Faraday component 2, polarizers 3 and polarizer holders (hereinafter referred to as a "holder") 5. Holders 5 are disposed at opposite axial ends of the magnetic member 1. Each holder 5 contains a respective polarizer 3. The polarizers 3 are so disposed that an angular difference of the planes of polarization thereof is forty-five (45) degrees around the optical axis.
FIGS. 19(a) and 19(b) are sectional views of typical conventional holders. The holder 5 in FIG. 19(a) is a circular cap with a recessed cross section and a slanted inner bottom surface inclined to the center axis thereof. Accordingly, the polarizer 3 fixed at the bottom of the holder 5 also has a certain amount of slant or incline to the axis. The holder 5 in FIG. 19(b) has a hollow cylindrical shape unlike the holder for the isolator shown in FIG. 18. The holder 5 in FIG. 19(b) holds the polarizer 3 in the hollow portion thereof. The polarizer 3 is connected to the inner surface of the holder 5 so that the polarizer 3 may have a certain amount of slant or incline to the center axis thereof.
FIG. 20 illustratively shows the installation of the polarizer 3 into the holder 5 in FIG. 19(b). In this example, a jig 7 with a cylindrical projection 7a mating with the hollow portion of the holder 5, is used for installation of the polarizer 3 into the holder 5. The holder 5 is placed around the projection 7a and the polarizer 3 is put on the top surface of the projection 7a. Then, the polarizer 3 and the holder 5 are fastened together. For fastening, various methods and materials may be employed, such as fastening with organic adhesive, solder or bonding glass. As described above, it may be common to place optical elements, for example, the polarizer 3, with a certain amount of slant or incline to the optical axis so as to avoid disturbance to the incident light by direct reflection from the element. In this case, the projection 7a may be so formed that the top surface thereof has a certain amount of slant or incline to the center axis of the holder 5.
Considering these structures of conventional optical isolators, the isolator in FIG. 17(a) may have a shortcoming in that it is awkward to dispose and fix the Faraday component 2 in the hollow portion of the cylindrical magnetic member 1, and thus is not suitable for mass production. Particularly, when the design policy of the isolator is strongly oriented to miniaturization, it may become difficult not only to assemble the elements but also to form a cylindrical magnetic member 1. For example, in the case of forming a miniature cylindrical magnetic member, the outer diameter of which is not more than five (5) millimeters, relatively expensive ways of manufacturing such as wire cutting must be employed instead of ordinary press working. Thus, mass production may not be easily achieved.
As for the structure shown in FIG. 17(b), because the Faraday component 2 is placed between the cylindrical magnetic members 1a and 1b, magnetic flux generated by the magnetic members 1a, 1b may not be efficiently used. As a result, stronger and larger magnetic members than those in other structures must be employed. As a result, optical isolators of this type cannot easily be miniaturized.
On the other hand, the holder 5 described in FIG. 19(a) may have following problems. Because the optical surface, i.e. the surface through which a light ray passes, of the polarizer 3 also constitutes a surface to be mounted to the holder 5, the optical surface is sometimes damaged by contact with the holder 5. The optical surface of the polarizer 3 also may be contaminated by bonding material such as adhesive agent fastening the polarizer 3 to the holder 5. Furthermore, because the length of the holder 5 is increased due to thickness of the inner bottom of the holder 5, the optical isolator including this type of holder may not be sufficiently miniaturized. In addition, it may be less effective to manufacture the cap-shaped holder 5 having a slanted or inclined inner bottom surface because of manufacturing costs.
In FIG. 19(b), the holder 5 does not have a positioning portion for the polarizer 3. Therefore, it may be difficult to support the polarizer 3 at a predetermined position so as to maintain a certain angle of incline to the optical axis. Thus, assembling efficiency may not be sufficient.